"It would be speculation to assess why God makes such capricious decisions."
Well, that statement already contains an assessment and leads to another---that the sign of the covenant is an example both of the capriciousness of God and His intent to shame His very own creation.
I wouldn't believe in a god with those attributes either.
Once you decide God doesn't exist, you close a door and decide never to walk through it again. At that point, your ability to understand what is on the other side of that door is forever compromised---although you can always take the stance that anyone who has chosen to walk through that door and describe whatever he has found there could not possibly either exist or make sense if it did.
Those of us who decide that God does exist open the door, close it with conviction, and choose to learn everything we can about what's on the other side. As we learn, we get insights and understandings about the reality on the other side that is unavailable to those who have not joined us there---or in some cases and for their own reasons have chosen to return to whence they came.
And that is the primary difference between an atheist and a theist---the theist cannot ever explain divinity to the atheist's satisfaction and the atheist cannot convince the theist that divinity does not exist.
But, Pea Nuts, here is one man's opinion. He seems slightly embarrassed in retrospect by his answer, but I find it interesting:
(And Abraham was 99 years old when God told him this was the sign of the covenant.)